Page 55 - Geochemistry of Oil Field Waters

TITRIM ETRIC METHODS 43

Calcu la t ions :
B x M x 100,100
sample volume = mg total hardness as CaC03 per liter

A x M x 40,000
sample volume = mg Ca per liter

(B-A) x M x 24,300
sample volume = mg Mg per liter
where A = calcium titer (blank); B = hardness titer (blank); and M = molarity
of CDTA.
The complexometric determination of calcium and magnesium will give a
precision of about 2% of the amounts present. The accuracy is dependent
upon the interferences present, and the major interferences are strontium and
barium, both of which will be complexed along with calcium, thus producing
high results. In the absence of strontium and barium the accuracy of the
method is about 4% of the amount of calcium and magnesium present.

Ammonium nitrogen

Organic compounds containing nitrogen decompose in a reducing environ-
ment and form ammonia and the ammonium ion. A reducing environment is
characteristic of a petroleum genetic environment (Collins et al., 1969).
Bogomolov et al. (1970) call it an indicator of petroleum.
The ammonium ion is too weak an acid to be successfully titrated;
however, when treated with formaldehyde, hexamethylenetetramine and a
strong acid are produced. This strong acid can be titrated with a base using
indicators or a potentiometer to determine the endpoint.

Reagents. The necessary reagents are hydrochloric acid, 12N; sodium
hydroxide standard, OJN, and 0.02N; and formaldehyde.

Equipment. The necessary equipment includes an expanded-scale pH meter,
a hotplate, microburets, flasks, and an ice-water bath.

Procedure. The method should not be used if less than 5 mg/l of ammonium
nitrogen is present. Acidify the brine or water when sampling to a pH of
about 1.5 with 12N HC1. The acid will stabilize the sample by changing to
the ammonium ion any ammonium hydroxide which could volatilize as
ammonia and be lost. Transfer a 100-ml aliquot of the acidified sample to a
250-ml Erlenmeyer flask and boil the solution for 5 minutes on a hotplate.
Cool the solution as quickly as possible to about 25°C. An ice bath will
facilitate .rapid adjustment to this temperature.